Development of the central nervous system (CNS) includes events of histogenesis, differentiation, migration, cell interactions (including afferents, targets, surrounding cells) and cell death. Many individual factors play key roles in CNS development including extracellular matrix molecules, trophic factors, cytokines and individual cellular components. The interaction of factors and cells insures that the above developmental events lead to development of a functional, mature CNS. Studies of these events contribute not only to our understanding of developmental processes, but also lay the foundations for understanding pathological conditions, especially those that occur in disorders of the pediatric CNS. With regard to development, chick spinal motoneurons have been well characterized, especially with regard to their differentiation, maturation and cell death. As a result, this model appears ideal to examine possible biochemical and molecular mechanisms that underlie motoneuron dysfunction and death in developmental disorders such as the Spinal Muscular Atrophies (SMAs). In the investigators' studies of the molecular mechanisms mediating motoneuron death during development, they have discovered that one of the messages that is up-regulated in motoneurons deprived of trophic support is APP. Furthermore, APP serves as a substrate for Caspase 3, one of the proteases that is activated in dying motoneurons. Additionally, inhibition of this cleavage prevented production of potentially toxic beta-amyloid (Abeta). This was the first evidence for a direct interaction between these molecules that has been subsequently confirmed by other laboratories. Additionally, their data also suggest that APP's involvement in neuronal death is the consequence of a more physiological role for APP and Abeta in regulating interaction between cholinergic motoneurons with their muscle targets during development. While recent reports indicate that both molecules are involved in Alzheimer's Disease, their results suggest an intracellular mechanism by which APP, caspases and Abeta mediate neuronal death during development and possibly in pediatric neuropathologies. Experiments in this proposal are designed to examine the appearance and function of APP and Abeta in the well characterized developmental system of chick spinal motoneurons. These results have the potential to provide novel insights into the molecules that contribute to motoneuron death underlying the SMAs.